Cycle-resolved visualization of lubricant-induced abnormal combustion in an optical natural gas/hydrogen engine

Abstract

The carbon-free policies and the shortage of crude oil have made engines fueled with natural gas get more and more attention, but actual engines fueled with natural gas often suffer from low engine power output and thermal efficiency. Hydrogen addition and high boosting are effective ways to improve natural gas engines’ efficiency and power output, however, abnormal combustion may occur and prevent the further increase in engine performance. In this study, the inner mechanism of lubricant-induced abnormal combustion was studied based on an optical natural gas/hydrogen engine. The test fuel was chosen as M70H30 and the in-cylinder lubricant was achieved by smearing the piston with excessive lubricating oil. Simultaneous pressure acquisition and high-speed direct photography were used to study flame propagation and abnormal combustion processes. The results show that the lubricant-induced auto-ignition occurred randomly, while no knock occurs due to the low energy density of M70H30. Besides, the abnormal combustion is observed to be directly related to the retained hot spot, and an early auto-ignition always results in too much negative work and low thermal efficiency. Comparing the location of abnormal combustion, it is found that the effect of auto-ignition on combustion also strongly depends on the location of hot spots. Furthermore, the comparison between end-gas auto-ignition cycle and normal cycle implied that non-knocking end-gas auto-ignition is favorable for improving natural gas/hydrogen engines’ efficiency, because the heat release is more concentrated. The current study shall give insights into understanding the abnormal combustion in high-power natural gas/hydrogen engines.